Dry planography, or waterless printing, is well known in the art of lithographic offset printing and has several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Careful ink water balance is unnecessary, thus reducing rollup time and material waste.
An unexposed waterless printing plate typically comprises a layer of ink repellent material over a layer of ink accepting material or an ink accepting surface. Because of their low surface energies and their ability to swell in the long-chain alkane solvents used in printing inks, silicone rubbers, such as poly(dimethylsiloxane) and other derivatives of poly(siloxanes), have long been recognized as preferred waterless-ink repelling materials. Preparation of the printing plate involves the imagewise removal of the ink repellent silicone rubber to expose the underlying ink accepting material or surface.
Various methods of removing the silicone rubber layer have been developed. Imaging with infrared lasers has been described by, for example, Eames, Canadian Patent 1,050,805, and by N. Nechiporenko and N. Markova, "Advances in Printing Science and Technology," Proceedings of the 15th International Conference of Printing Research Institutes, Lillehammer, Norway, June 1979, Pentech Press, London, p. 139-148. The silicone rubber layer is coated over an absorber layer containing a infrared absorbing material in nitrocellulose. Imagewise exposure with an infrared laser partially disrupts the absorber layer, allowing it and the overlying silicone layer to be removed from the exposed regions with a solvent. Infrared imaging has also been described by Lewis, U.S. Pat. Nos. 5,310,869; 5,339,737; 5,385,092; and 5,487,338.
In each these methods, mechanical wiping and/or washing with liquids was required to remove the silicone rubber after exposure. Wiping has several drawbacks. It is difficult to reproducibly remove all stray material with automated cleaning stations. Wiping can scratch and/or abrade the printing plate.
A processless printing plate, i.e., one that does not require a separate processing step to remove the silicone rubber after imaging, would have several advantages. The development step would be eliminated, simplifying the process for preparing the printing plate. If desired, the plate can be exposed on the printing press, which would eliminate damage to the plate caused by handling and mounting on the press after imaging. In addition, any scratching or abrading the plate surface caused by development would be eliminated.
There are three key requirements for an ink repellent polymer to be useful for a thermally-imageable, processless waterless printing plate: the polymer must form a solid film at room temperature to resist damage from the press, it must release ink, and it must be easily removed by the imaging step or by the normal action of the press after imaging. The need for a development step arises from the conflicting need to have wear resistant layers for long press runs while maintaining ease of removal by heat.
In the uncrosslinked form, silicone polymers are either fluids or gums and lack the physical properties needed for handling and printing. Therefore, silicones are generally crosslinked by a number of methods including reactions between silicone hydride and Si-vinyl, reactions between Si--OH or Si--OR groups, and other well known crosslinking chemistries. Although these crosslinks impart robust physical properties to the film, the cross links are not readily broken down by heat, making thermal imaging difficult. A thermally exposed film retains its integrity and is not altered enough to be easily removed. Silicone debris clings to the substrate and to background areas and must be physically wiped away. Polymers with greater thermal sensitivity are required.
A need exists for a thermally-imageable, processless waterless plate in which the ink repellent layer is a polymer that is solid, wear resistant material, but is easily removed either by the imaging step or by the normal action of the press after imaging.